Sun

The Sun is the star around which the solar system revolves.

• Diameter: 1392600 km, 865400 mi, 109 times Earth's.
• Mass: 1.9885 × 10³⁰ kg, 333k times Earth's.
• Luminosity: 3.828×10²⁶ watts.
• 99.86% total mass of the solar system.
• ~75% hydrogen by mass.
• The rest is helium except for less than 2% metals (one determination is 1.22% metals).
• main-sequence G-type star.
• 4.6 billion years old.
• Roughly half through its expected stable life (main-sequence lifetime).
• About 25 kly from the galactic center.
• In the Milky Way, within the Local Group, within the Virgo Supercluster, within the Laniakea Supercluster.

Compared to other stars:

• -26.74 - apparent magnitude.
• +4.83 absolute magnitude.
• 0.0000158 ly - distance from Earth.
• G2V - spectral type.
• +0.656 - estimated B-V color index.

The study of stars reveals information about the Sun's past and future, and the study of the Sun reveals characteristics that other stars are presumed to have. Aside from evidence for basic stellar structure the Sun shows granulation, constantly changing light and dark spots (called granules) on the order of 1500 km in diameter, and supergranulation, larger (5000-30000 km) slower changing regions, the latter assumed to reflect the regions of convection.

Early main sequence Sun (ZAMS and soon after):

• 75% of the luminosity of today's Sun.
• ~90% of the current solar radius.
• cooler.
• 3-day rotation period.
• stronger magnetic field, hotter chromosphere, more X-rays and extreme ultraviolet.
• flares 100 times more energetic.

Future Sun:

• Luminosity will roughly double while in the main sequence.
• Eventually will become a red giant.
• Later: the horizontal branch.
• Later: the asymptotic giant branch.
• Later: planetary nebula.
• Later: white dwarf.

Solar values often used as physical units:

• R_☉, solar radius, R_Sun, or Rsun - radius of the Sun.
• M_☉, solar mass, M_Sun, or Msun - mass of the Sun.
• L_☉, solar luminosity, L_Sun, or Lsun - luminosity of the Sun.

Pedantic note on capitalization: I find a capitalization rule stating "Sun" is capitalized when it used as the name of a star, e.g., within a list of stars. For this astrophysics site, I make it easy on myself, interpreting this rule broadly and virtually always capitalizing the word: I only use "sun" when the word is used in the sense of "sunshine", such as "I like to be out in the sun". My practice does not follow Wikipedia and much astronomical literature, which generally do not capitalize the word.